1. Field of the Invention
The invention relates to aiming apparatuses having a plurality of units operating in different ranges of the optical spectrum and it is particularly suitable for use in sighting apparatus for weapon systems.
2. Prior Art
Aiming apparatuses are widely used which include a detector sensitive to radiation in the visible range (sighting telescope for visual view or television camera), a detector sensitive to the far infrared (thermal camera), and a laser designator or range finder working in the near infrared. It is necessary to achieve mutual or boresight alignment of the optical paths of the different units. This operation, sometimes termed harmonization, requires a common collimated optical reference emitting in the different sensitive ranges.
A solution which comes immediately to mind consists in using a retractable mirror for directing light emitted by a source-forming hole having a sufficient brightness in the different ranges of the spectrum, when required, to the different detectors. The retractable mirror, depending on its position, directs the light from the source hole to the detectors or clears the field of view thereof. This solution has proved unsatisfactory. It is difficult to provide a source capable of homogeneously illuminating the hole in the different ranges of the spectrum. If a halogen lamp is used whose filament emits in the visible spectrum and whose bulb emits in the infrared spectrum, a dual problem is met with: since the filament and the bulb are at different distances from the source hole, illumination is not homogeneous. Moreover, for numerous applications and particularly on vehicles, the filament is rapidly destroyed by vibrations. It is not possible to adjust the brightness independently for the different spectrum ranges under consideration. Finding a material which is transparent for the whole of the spectrum range to be transmitted and fulfils the field requirements is a problem. A number of other arrangements have been constructed and are described in U.S. Pat. No. 4,422,758 (Godfrey et al.). None is fully satisfactory for use on a vehicle and with detectors working in widely spaced ranges.